The invention relates to systems and methods for communicating with wireless devices.
Many systems today use wireless devices to transmit information. Television remote controls transmit information to a television via infrared signals. Cellular phones use radio frequencies to send and receive information. Wireless systems for monitoring inmates in a location employ the tight coupling of a transmitter on an inmate to the center point receiver in order to alert a “break out” condition when the inmate wanders outside the allowed radius of the receiver. In order for any transmitter in such systems to communicate with it's corresponding receiver, that receiver needs to know intimate knowledge about the types of transmitters attempting the communication, the protocols those transmitters are using, and specific instance information about the particular transmitters.
In a first example, television remotes are made for and are hard wired for a particular television or class of televisions that those remote controls control. The knowledge in this system about the types of remote control transmitters that the receiver television expects is intimately bundled together since the television remote comes in the same package as the television. There is also an implicit understanding that the receiver television will understand the remote control transmitter bundled with that television. The knowledge about the protocols used by the remote control to communicate with the television are hard-wired into the remote as specific infrared signals that are sent when certain buttons are pressed. The television knows what to expect because it is hard-wired to receive and process a specific set of signals. As such, only a remote made for that particular television or class of televisions is able to communicate with that television and is able to be used to control that television—with the caveat that learning remotes, universal remotes, and the like, can mimic the particular remote in question. In such a system, registration is not necessary as the hard-wired nature of the transmitter and receiver programmed at the factory serve as the only registration necessary. Such a level of simplicity is desirable in certain systems, but the flexibility and lack of transmitter uniqueness in such a system is undesirable.
As a second example, a cellular telephone operates in a very different manner. Cellular telephones work on the principle that each specific instance of a telephone handset is different, and the exact identification of particular instances of a cellular telephone is critical. A cellular provider only wants individuals registered with their services and in good standing to use their networks. Not every cellular telephone is able to or should be allowed to use the network. The electronic subscriber number, or ESN, is used at registration time to identify cellular phones within the system. In fact, typically, a user must call the service provider and verbally provide the ESN in order to be allowed access to services on that service provider's network. Each time a call is placed, the ESN serves as the authentication token to gain access to the network. The ESN is stored on the service provider's system and compared against the one sent by the phone when a call is made or when heartbeat event occurs.
The particular protocol used can then be programmed into the phone at registration time or may have already been programmed into the phone earlier. Such a system creates an ownership and privilege relationship between the phone and the service provider which can be crucial since increasingly large numbers of individuals make use of wireless devices in close proximity to one another. As such, it is important to prevent spurious devices from gaining access to a user's local domain. The identification of a wireless transmitter with it's associated receiver through registration, as in cellular phone registration, is the solution. However, such a process can be cumbersome enough for one phone and is especially burdensome when a user needs to register ten, or even one hundred, phones.
In a third example, The wireless inmate tracking devices are a hybrid of the above two concepts. Like cell phones, these devices can use an authentication scheme similar to the ESN scheme employed by cell phones. These wireless monitoring systems transmit a packet of information at a designated time interval in order to note the existence of the inmate. If an appropriate packet is received within the time interval specified, then the inmate is deemed to be in the acceptable radius while the absence of a regularly scheduled packet signifies the possible escape of such an inmate. To avoid the possibility that a person could use a similar device to mimic their tracking device, and, thus, escape under the guise of perceived existence, an authentication token can be used that is similar to an ESN. The affinity gained by such an ESN-like scheme captures the unique relationship necessary to correctly identify and distinguish the presence of the inmate within the allowable radius. The registration for such devices, however, are often hardwired by the manufacturer, the manufacturer burning in the identification information on both the transmitter and the receiver and shipping both transmitter and receiver bundled together. Such devices possess the ease of use inherent to television remote controls which do not require unique identifiers, but they still suffer from the requirement that identifying information must be hard-coded by the manufacturer.
Thus, there exists a need to have a system in which the registration of the transmitter devices is flexible and sufficiently easy to perform by a user, which can be performed repetitively without becoming burdensome. In addition, there exists a need for the system which, at the same time, guarantees the uniqueness of the transmitter devices and allows access restrictions necessary for users to effectively manage their local domain of devices. It is therefore an object of the present invention to provide improved systems and methods for registering and authenticating wireless devices.
Another problem that frequently arises in many wireless receiver/transmitter systems is the problem of clock drift between the controller 28 (the receiver) and the wireless control devices 210 (the transmitter). This is especially the case where inexpensive asynchronous transmitters and/or receivers are utilized that have inexpensive RC oscillators or the like that serve as the clock. Typically, these RC oscillators are unstable clocks and require periodic resynchronization to be useful. What's more, the slight variation in the current drawn from batteries, which many of these wireless control devices use, frequently cause the oscillators to run at slightly skewed rates as compared one to another. As such, these inexpensive receivers and transmitters which have unstable clocks are prone to drifting out of synchronization over time, causing errors in the data that often cannot be recovered from.
Because a data bit exists as either a high voltage signal or a low voltage signal, for encoding a 1 or a 0, and because that voltage is changed every regular time interval, clock drift can quickly cause the transmitter and the receiver to think that one is sending one bit while the other is reading another bit. In a transmitter/receiver system with a one second clock drift after five seconds and a one second time interval serving as the regular window in which a bit is encoded, after five seconds a receiver would be out of sync by an entire bit after 5 bits, causing the receiver to think that it is reading either the sixth or the fourth bit after the first five seconds. It is clear that such a data read would be erroneous.
One solution is to employ the use of highly synchronized crystals to keep a regular time in the transmitter and the receiver. For several reasons, this is undesirable generally. For fairly “inexpensive” devices, such as a light switch or a smoke detector, the grade of synchronized crystals needed to keep the clock drift problem at bay can result in a significant cost of the manufacturing of such “inexpensive” devices going to the cost of the crystals. Moreover, the data pins on a microprocessor in “inexpensive” devices are often in short supply as product manufacturers squeeze more functionality out of these “inexpensive” devices. A crystal can take up more than one pin while more traditional systems, perhaps using the clock inherent in battery-based clocks, take up only one pin on the microprocessor.
Therefore it is another object of the present invention to find a scheme which allows devices employing “inexpensive” oscillators to be used in a way which will allow arbitrarily long multi-byte transmissions in the face of clock drift.